They don't adapt well to Nikon DSLRs, but loads of other cameras can make use of them - not as many as will take M42 but PB is still readily adaptable.
I've got the M42 version of the 50mm & possibly the 135. I don't think I have that 35, but I do have the CZJ Flektogon 35mm/2.4 which is a truely fantastic lens.
I don't think they'll sell for much but you should be able to get at least £20 for each lens. Strangely I've found cameras with a lens attached often go for less than the lens alone.